q + search at leps/spring-8

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+ Search at LEPS/SPring-8T. Nakano

（ RCNP, Osaka University）

Exotic Hadron WS @ NWU, May 27, 2005

• Introduction• Evidences for +

• Counter-evidences for +

• Preliminary results from new LEPS data• Summary

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c.f.(1405): uudsu or uds

e.g. uuddc, uussd

What are penta-quarks? Baryon. Minimum quark content is 5 quarks. “Exotic” penta-quarks are those where the antiquark

has a different flavor than the other 4 quarks Quantum numbers cannot be defined by 3 quarks

alone.

Qqqqq

Example: uudds

Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1

Strangeness = 0 + 0 + 0 + 0 + 1 = 1

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Prediction of the + Baryon

M1890-180*Y] MeV

D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305.

• Exotic: S=+1

• Low mass: 1530 MeV

• Narrow width: ~ 15 MeV

• Jp=1/2+

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Baryon masses in constituent quark model

• Mainly 3 quark baryons:M ~ 3Mq + (strangeness)+(symmetry)

• 5-quark baryons, naively:M ~ 5Mq + (strangeness) +(symmetry)1700~1900 MeV for +

mu ~ md = 300 ~ 350 MeV, ms=mu(d)+130~180 MeV

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•DPP predicted the with M=1530MeV, <15MeV, and Jp=1/2+.

•Naïve QM (and many Lattice calc.) gives M=1700~1900MeV with Jp=1/2-.

•But the negative parity state must have very wide width (~1 GeV) due to “fall apart” decay.

Theory

For pentaquark

Fall apart

Ordinary baryons

qq creation

•Positive parity requires P-state excitation.

•Expect state to get heavier.

•Need counter mechanism.

diquark-diquark, diquark-triquark, or strong interaction with “pion” cloud?

Positive Parity?

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Evidence for Penta-Quark StatesSpring8 ELSA

JLab-p

HERMES

ITEP

pp ++.

COSY-TOFDIANA

SVD/IHEP

JLab-d

ZEUSCERN/NA49

H1

Nomad

This is a lot of evidence

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Background level was estimated by using events from a LH2 target.

Assumption:• Background is from non-resonant K+K- production off the neutron/nucleus• … is nearly identical to non-resonant K+K- production off the proton

First evidence from LEPS nK+K-n

peak in events without a proton.

1.540.01 MeV< 25 MeV

Phys.Rev.Lett. 91 (2003) 012002

hep-ex/0301020

+

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Final state:

Ks + p

K+ + n

(Ks + p )

Mass

A few % difference from zero, but ~20% difference from the KN threshold.

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Width

• There is some inconsistency:– Most measurements are only upper limits.– DIANA has < 9 MeV. – The cross-section implies =0.9 MeV.– HERMES: = 13 +- 9 stat. (+- 3 sys.) MeV– ZEUS: = 8 +- 4 stat. (+- 5 sys.) MeV– Arndt et al. and Cahn et al. analysis of KN phase

shifts suggests that < 1 MeV !!

• The small width is the hardest feature for theorists to understand…

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• HERA-B (Germany):– reaction: p+A at 920 GeV– measured: K-p and K0p invariant mass– Clear peak for (1520), no peak for +

– production rate: +/(1520)<0.02• BES (China):

– reaction: e+e- J/ +-

– limit on B.R. of ~10-5

Negative Results

And many unpublished negative results

(HyperCP, CDF, E690, BaBar, LEP,,,).

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+ Search at CDF: pK0S Mass Spectrum+ Search at CDF: pK0

S Mass Spectrum

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Slope for mesons

Slope for baryons

Slope for pentaquarks??

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CLAS: New high statistics exp.Search for + in pK+Ksn

R. De Vita, APS April meeting, 2005

Limit on + production:Total cross-section < 1~4 nb

How could other low energy experiments with lower statistics could observe the signal?

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LEPS New LD2 and LH2 runs

• Data taken from Oct. 2002 to Jun. 2003.

• ~2 ・ 1012photons on a 15cm-long LD2 target.

•Less Fermi motion effect.

•LH2 data were taken in the same period with ~ 1.4 ・ 1012photons on the target.

#of photons: LH2:LD2 ≈ 2:3

we expect

# of events from protons: LH2:LD2≈ 2:3

# of events: LH2:LD2≈ 1:3

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Search for + in nK+K-n

MM (GeV) MM (GeV)

•A proton is a spectator (undetected).

• Fermi motion is corrected to get the missing mass spectra.

•Tight exclusion cut is essential.

•Background is estimated by mixed events.

preliminary

preliminary

L(1520)

pK+K-p nK+K-n

Can be consistent with the CLAS high stat result?

Next talk by S.I. Nam

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LEPS detector

1m

Good acceptance in the forward angle.

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New way to search for +

Θ ＋ is identified by K － p missing mass from deuteron. ⇒ No Fermi correction is needed.

γ

p

n

Θ ＋

K －

p

γ

p

n

Θ ＋

K －

p

(1520)

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A possible reaction mechanism• + can be produced by re-scattering of K+.• K momentum spectrum is soft for forward going (1520).

γ

p/n

n/p

(1520)

K+/K0

PK obtained by missing momentum

Formation momentum

LD2

PK GeV/c

MM

d(γ

,K－

p)

GeV

/c2 LH2

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Final State Interaction

γ p

K+

K-

γp

K+

K-

p

p

n

nn

n

K-

K+

γ

pK+

pn

n

K-

p

γ

p

K+

K-

pn

n

K+

1520)

Large at low PpSmall, especially at low PK+

I.

II.

III.

Contributions from II and III are suppressed.

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Event selection

MMp(γ,K － p) GeV/c2 M(K － p) GeV/c2

Λ(1520)

Select Λ(1520) in 1.50–1.54 GeV/c2

　　　　　 ⇒ calculate K- p missing mass of d K- p X reaction

K mass is smeared by Fermi motion. (assumed proton at rest)

LD2 data:

after selecting (1520)

inelastic events

select

LD2

γp→K － pKπ

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K － p missing mass in 1.50<M(K － p)<1.54 GeV/c2

MMd(γ,K － p) GeV/c2

1.53 GeV d (1520) X K- p

Good understanding of the background spectrum shape is crucial .preliminary

“You see the peak because you want it.

I see two dips.”

by Prof. I at KEK

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Major background process

• Quasi free (1520) production must be the major background.

• The effect can be estimated from the LH2 data

γ

p

n

(1520)

K+

n

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contribution

M(K － K+) GeV/c2

K+ momentum is estimated by missing momentum technique. Smeared by Fermi motion.

LD2 data

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K － p invariant mass after exclusion

M(K － p) GeV/c2 M(K － p) GeV/c2

E > 2.2 GeVall energy

LH2 LH2

non-resonant KKp events

non-resonant KKp events

Non-resonant KKp (phase space) contribution reproduces the spectrum shape under L(1520) well.

(1520)

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K － p missing mass for (1520) and non-resonant events in the

MMd(γ,K － p) GeV/c2 MMd(γ,K － p) GeV/c2

non-resonant KKp (1520)

Energy dependences are set to be the same.

No angular dependence has been introduced in the both reactions.

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K － p missing mass for (1520)


Quasi-free and non-resonant KKp

preliminary

preliminary

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K － p missing mass in the sideband regions

MMd(γ,K － p) GeV/c2 MMd(γ,K － p ) GeV/c2

1.46< M(K － p) < 1.50 GeV/c2 1.54< M(K － p) < 1.58 GeV/c2

: Energy and angular dependences were obtained by fitting to the data.

KKp: Enegy dependence fit to the data. Angulardependence flat.

LD2 LD2

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Test by side-band subtractionNarrow gate (1.52<M(K － p)<1.54 GeV/c2) = L + N Λ(1520) is enhanced.Wide gate (1.46<M(K － p)<1.58 GeV/c2) = L + 3N Non-resonant KKp is enhanced.　　　　　⇒ L = 1.5 (L+N) － 0.5 (L+3N)　　　　　　　　　　　　 narrow gate wide gate

M(K － p) GeV/c2

L

NNN

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K － p missing mass for non-resonant KKp events (phase space distribution)


1.46< M(K － p) < 1.50 GeV/c2

1.50< M(K － p) < 1.54 GeV/c2

1.54< M(K － p) < 1.58 GeV/c2

sidebands averaged

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Side-band subtraction in KK invariant mass

M(K+,K-) GeV/c2 M(K+,K-) GeV/c2

after side-band subtraction

Since φ is not related with Λ(1520),φ peak disappears.

LD2

LD2

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K － p missing mass in(1520) and sideband region.


LD2 LH2

preliminary preliminary

1.50 < M(K － p) < 1.54 0.5(1.46 < M(K － p) < 1.50) +0.5(1.54 < M(K － p) < 1.58)

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Side-band subtracted K － p missing mass


Excess is related with Λ(1520).

Bump structure

LD2 LH2

preliminary

preliminary

Most events are below 1.52 GeV/c2

Events in MMd(γ,K － p) < 1.52 GeV/c2: LH2:LD2 = 1:1.3

# of photons: LH2:LD2 ≈ 2:3 (p) >> (n) for quasi free (1520) production.

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Energy dependence


LD2 LD2


1.75 GeV < E < 2.1 GeV 2.1 GeV < E < 2.4 GeV(Emax)

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K － p missing mass in high and low energy region


preliminary

Below 2.1 GeV

Above 2.1 GeV

1.53 GeV Peak:•No change in the peak position. not likely due to kinematical reflections.•Narrower width in the low energy region. consistent with the detector resolution.

1.60 GeV Bump:•Only seen in the low energy region. threshold effects?•Not seen in LH2 data•Associated with (1520).

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contribution removed by angle cut


LD2 LH2


Require cos Kp > 0

Excess remains.

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K － p missing mass in sideband regions


γ (1520)

K+/K0

γ p

K+/K0

K-

No peak!

+ formation cross-section by simple kaon re-scattering should be small.

LD2

A theoretical estimation by A. Titov is small.

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Width: Comparison with a MC spectrum


MC gives ~10 MeV resolution while Real data resolution is about ~ 8 MeV.

The statistical uncertainty of ~2 MeV is mainly due to fluctuations of the background at the tails of the peak.

preliminary

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Summary• Evidence for an S=+1 baryon around 1.54 GeV

with a narrow width has been observed by several experimental groups.

• There are some inconsistencies in the measured masses and widths.

• No signal has been observed in high energy experiments with high statistics and good mass resolution.

• If the + does exist, its production in high energy reactions must be highly suppressed.

• The + is not seen in the new CLAS high statistics data from a proton.

• If the + does exist, its production must have a large isospin asymmtery.

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Summary of LEPS new result• Peak is seen at ~1.53 GeV/c2 in the missing mass of the (,(1520)) reaction from a deuteron.• If is is real, the width seems to be very narrow.• The peak cannot be seen in the K-p invariant mass region outside of the (1520). • If the peak is due to the +, its production by re-scattering seems to be small in our kinematic region.• Enhancement (bump structure) around 1.6 GeV is also observed in the (,(1520)) reaction but only in the low E region.•Non-resonant KKp and contributions can be removed by sideband subtraction.•For quasi-free (1520) production, (p)>>(n).

q + search at leps/spring-8

Documents

q production

q baryonm

q l1520q search

q production rate

q preliminary results

q counterevidences

p g nk

leps new ld2